1. Field of the Invention
The present invention relates to a light receiving device and an encoding device, and particularly, to a light receiving device including plural light receiving elements and an encoding device thereof.
2. Description of the Related Art
In the related art, an encoding device is used to detect linear motion or rotational motion of an object, and input the detection results to a computer or other digital devices. For example, the encoding device generates pulses corresponding to the motion of the object to make the detection.
Usually, the encoding device includes a member moving in a direction in which several openings are arranged, light receiving elements that detect light passing through the openings, and a signal processing unit that generates the pulses from the output signals of the respective light receiving elements. For example, Japanese Laid Open Patent Application No. 6-18290 discloses such a device.
FIG. 6A and FIG. 6B are a plan view and a side view illustrating an encoding device in the related art.
As shown in FIG. 6A and FIG. 6B, an encoding device 1 includes a light source 10, a slit member 11, and a light receiving unit 12.
The light source 10 and the light receiving unit 12 are fixed and are separated at a certain distance. The slit member 11 is arranged between the light source 10 and the light receiving unit 12, and is movable relative to the light source 10 and the light receiving unit 12 in X1 and X2 directions (the right side or the left side in the X direction in FIG. 6B). The slit member 11 is fixed to an object to be detected, and moves in the X direction when the object moves.
The light receiving unit 12 includes adjacent light receiving elements 12a, 12b, 12c, 12d arranged in the X direction. For example, the light receiving elements 12a, 12b, 12c, 12d are photo diodes. Each of the light receiving elements 12a, 12b, 12c, and 12d has a length dx1 in the X direction and a width dy1 in the Y direction, and the light receiving elements 12a, 12b, 12c, 12d have the same light receiving area.
The slit member 11 includes light transmitting parts 11a and light shielding parts 11b which are arranged alternately and repeatedly. Each of the light transmitting parts 11a and the light shielding parts 11b has a length dx2 in the X direction and a width dy2 in the Y direction, and dx2 and dy2 satisfy dx2=2×dx1, and dy2=dy1+α, that is, the width dy2 of the light transmitting parts 11a and the light shielding parts 11b is greater than the width dy1 of the light receiving elements 12a, 12b, 12c, 12d by α.
The output signals from the light receiving elements 12a, 12c are compared in a comparator 13, and the comparison result is output through a terminal 14 as a detection signal.
The output signals from the light receiving elements 12b, 12d are compared in a comparator 15, and the comparison result is output through a terminal 16 as a detection signal.
When the slit member 11 is moved relative to the light receiving unit 12 in the X1 direction (to the right side in the X direction in FIG. 6B), the output signal of the comparator 15 is later than the output signal of the comparator 13 by ¼ cycle. On the other hand, when the slit member 11 is moved relative to the light receiving unit 12 in the X2 direction (to the left side in the X direction in FIG. 6B), the output signal of the comparator 15 is earlier than the output signal of the comparator 13 by ¼ cycle.
However, in the encoding device 1 in the related art, because the edges of the light receiving elements 12a, 12b, 12c, 12d are formed to be parallel to the edges of the light transmitting parts 11a and the light shielding parts 11b, the detection signals of the light receiving elements 12a, 12b, 12c, 12d have monotonically increasing or decreasing waveforms. For this reason, jitter of the detection signals is large.
In addition, the detection resolution of the encoding device 1 is determined by the width of the light transmitting parts 11a and the light shielding parts 11b of the slit member 11, and changes with the width of the light transmitting parts 11a and the light shielding parts 11b of the slit member 11. In order to reduce the width of the light transmitting parts 11a and the light shielding parts 11b of the slit member 11, special techniques are required, and this increases the cost of the slit member 11. Further, if the width of the light transmitting parts 11a and the light shielding parts 11b of the slit member 11 are reduced, the light intensity supplied to the light receiving elements 12a, 12b, 12c, 12d become insufficient, and this causes degradation of the detection precision.